1. Field of the Invention
The present invention relates to an optical receiver for an optical communication system, particularly an optical receiver that can be used advantageously in an optical wavelength division multiplexing (WDM) communication system that transmits signals by using light having a plurality of different wavelengths.
2. Description of the Background Art
As optical communication systems become prevalent in the world, they have been increasingly incorporating WDM transmission systems that use a plurality of wavelengths in order to further increase the capacity of communication. In particular, researchers and engineers have been studying a system that connects a central office and subscribers by using a plurality of wavelengths, for example from four to sixteen wavelengths, in order to introduce optical fibers into the subscribers' lines at low cost.
In this case, a high-performance wavelength multiplexer is placed at a wavelength multiplexing point at a ratio of m:1 (m: the degree of multiplexing) at a central office to produce light having multiplexed wavelengths. At the subscribers' side, on the other hand, a high-performance wavelength demultiplexer is placed at a wavelength demultiplexing point at a ratio of 1:m to separate the light according to individual wavelengths. Supposing that the light has multiplexed wavelengths consisting of λ1, λ2, . . . , λm, the wavelength demultiplexer selects light having a wavelength, λj, assigned to the j-th subscriber to send the selected light to the optical fiber for the j-th subscriber. In principle, any wavelength demultiplexer can also be used as a multiplexer. Thus, for simplicity, the word “multiplexer” is used in this specification as a general term to refer to both multiplexers and demultiplexers, except when it is necessary to distinguish the two devices or functions. Because high-performance optical wavelength multiplexers can multiplex and demultiplex a multitude of wavelengths at a time, they have already been put into practical use in long-distance, wide-area transmission systems.
In order to apply the above-mentioned WDM communication system to a small area, it is essential to construct the system at low cost. Conventional optical wavelength multiplexers placed at wavelength multiplexing points at a ratio of m:1 and at wavelength demultiplexing points at a ratio of 1:m are extremely high-cost, which as a result increases the cost of the WDM communication system. Consequently, researchers and engineers have been studying to develop a system in which no expensive optical wavelength multiplexers are used.
One of the studies has been reported by Y. Murakami and M. Kouchi in a report, entitled “Development of simplified WDM system—Development of compact EDFA and optical receiver provided with optical BPF,” in the Technical Report of IEICE OCS99-60 (1999–09) (pp. 41–46), where IEICE is a abbreviation for The Institute of Electronics, Information and Communication Engineers of Japan.
The WDM communication system in this report uses inexpensive star couplers, in place of optical wavelength multiplexers, at wavelength multiplexing points and wavelength demultiplexing points. Because a star coupler has no function of wavelength separation, it distributes light having the same wavelength spectrum to all the optical fibers connected to it. At a light-receiving side, a band pass filter (BPF) selects light having the wavelength λj to receive it.
In the system proposed in the report, it is advantageous to bring the wavelengths of a plurality of optical signals closer to one another so that the number of subscribers can be increased. However, when the neighboring wavelengths are close to each other, strict control over the accuracy of the wavelength of light produced by an optical source and over the accuracy of the wavelength of light received by an optical receiver is required. Consequently this increases the cost. In this report, the system is designed to use eight wavelengths in the 1.5 μm band, with each wavelength having a band width of 3.2 nm. More specifically, eight center wavelengths are assigned to, for instance, 1,536.6 nm, 1,539.8 nm, 1,543.0 nm, . . . , 1,559.0 nm at 3.2-nm intervals. Because the center wavelengths are assigned at extremely narrow intervals, the light-emitting devices must be laser diodes (LDs) having an extremely sharp spectrum in the emitted light at the assigned center wavelength. Similarly, at a light-receiving device side, it is necessary to use a wavelength-selecting filter having a sharp wavelength selectivity.
Some terms used in this specification are defined below. Supposing that the number of different wavelengths used in a WDM transmission system is m, the wavelengths are denoted in λ1, λ2, . . . , λm. Supposing that the same interval is given to the neighboring wavelengths, the interval is denoted in Δ. Of course, different intervals may be employed.
Supposing that the number of subscribers (optical network units: ONUs) is m, the subscribers are denoted in U1, U2, . . . , Um. The j-th subscriber, Uj, exclusively receive the j-th wavelength, λj=λ1+(j−1)Δ. Consequently, the wavelength λj is referred to as “the assigned wavelength” for the subscriber Uj. The set of the other wavelengths, λ1, λ2, . . . , λj−1, λj+1, . . . , λm, is referred to as “the complementary wavelength” and denoted in Γj. Hence, Γj={λ1, λ2, . . . , λj−1, λj+1, . . . , λm}. This can also be expressed in Γj=Λ−λj, where Λ signifies the set of all the wavelengths.
The j-th LD, which emits light having the wavelength λj, is denoted in Lj. The light emitted from Lj has a sharp peak at the wavelength λj. The band width assigned to each wavelength is Δ or less, which is extremely narrow. The j-th filter, Fj, selectively transmits the assigned wavelength λj out of the set of all the wavelengths, Λ={λ1, λ2, . . . , λm}, reflecting the complementary wavelength, Γj={λ1, λ2, . . . , λj−1, λj+1, . . . , λm}.
The system in the report uses a star coupler, which is low in cost and has no function of wavelength separation, for multiplexing and demultiplexing wavelengths. Consequently, an optical receiver must select the assigned wavelength on its own. However, photodiodes (PDs) based on InGaAs, conventionally used for optical communications, have high sensitivity in a wide wavelength spectrum from 1.0 to 1.6 μm. In other words, an InGaAs-based PD has no wavelength selectivity. Therefore, the system in the report places a BPF that selectively transmits the assigned wavelength forward of the PD in an optical receiver.
A BPF is produced by laminating a plurality of layers consisting of at least two types of dielectric layers having different refractive indexes and thickinesses. It transmits a specified wavelength only. The system in the report uses a BPF that transmits the assigned wavelength, λj, and reflects the other wavelengths, i.e., the complementary wavelength, Γj.
FIG. 1 shows a cross-sectional view of an optical receiver used in the report. A disc-shaped metal stem 1 mounts a PD 2. Although not shown in the figure, the stem 1 has pins, to which the upper and lower electrodes of the PD 2 are connected with wires. The stem 1 is securely inserted into a lens holder 4, made of cylindrical metal, that holds a lens 3 at the forward opening. The lens holder 4 is inserted into a larger cylindrical housing 5. A cylindrical holder 6 is welded to the end of the housing 5 in optical alignment.
A cylindrical inverse collimator 7, a disc-shaped BPF 8, and a cylindrical collimator 9 are inserted into the holder 6 in optical alignment. A cylindrical ferrule 11, also, is inserted into the holder 6 to support the end of an optical fiber 10.
Incoming light having multiplexed wavelengths has all the wavelength components (Λ={λ1, λ2, . . . , λm}). A beam of incoming light emerges from the optical fiber 10, is spread out by the collimator 9 to become a parallel beam, and is subjected to the wavelength selection carried out by the BPF 8. The BPF 8 transmits the assigned wavelength only and reflects the complementary wavelength. The transmitted light is focused through the inverse collimator 7, passes through an opening 12, enters the housing 5, is focused through the lens 3, and finally enters the PD 2. Thus, the PD 2 detects the assigned wavelength λj only.
As mentioned above, a collimator is provided in an optical receiver, because a BPF made of a dielectric multilayer film cannot perform strict wavelength selection unless a parallel beam of light is supplied. Even a slight gradient of a beam can cause an error in the wavelength selectivity. When light emerges from an optical fiber, the beam of light spreads out in accordance with the numerical aperture (NA) determined by the refractive indexes of the core and cladding. Divergent rays of a beam have different angles of gradient in the cross section. Therefore, the BPF cannot select the assigned wavelength accurately under this condition. It is necessary to use a collimator to introduce light in the form of a parallel beam into a BPF made of a dielectric multilayer film.
The optical receiver shown in FIG. 1 is large and extremely expensive because it is composed of a housing, a lens holder, and a holder to house and secure a lens, a collimator, and an inverse collimator. The optical receiver measures no less than 7 to 10 mm in diameter and 15 to 20 mm in length. It is difficult for a system that requires such large and expensive devices to come into widespread use as a communication system for connecting subscribers of general households.